It's a bracket thing, try this:

(0>(SELECT COUNT(*) FROM pos_s_invoice WHERE (doc_no IS NOT NULL and pos_is <> 899)GROUP BY doc_no HAVING (COUNT(*) > 1)))

>Hi Jon,
>
>thanks for you reply. I tried the code but i still have the same problem. In fact i don't see any difference between your code and mine except for the table and field names. Where are you inserting this code ?
>Is it when you do a right click on your table, than choose design table,right click check constrains and then new ?
>
>thnx